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STi + N2O

A late-model Subaru Impreza STi with a surprise!

Words by Michael Knowling, Pix by Julian Edgar

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At a glance...

  • MY03 Subaru Impreza STi
  • Upgrade turbo and bolt-ons
  • And a nitrous bottle hidden in the boot!
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The mere sight of this MY03 Subaru Impreza STi causes its owner, Jim, a painful combination of excitement and frustration. It’s exciting because it’s a nitrous-fed monster that spits out around 400kW. But it’s also a source of immense frustration because, well, it’s as useful as an mantelpiece ornament. You see, Jim recently lost his licence while driving his Subie. No longer can he wind out that hi-po flat-four through the top-end; heck, he can’t even drive to the shop for milk.

But disqualification from driving doesn’t mean abandoning your plans to modify!

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Open this STi’s aluminium bonnet and you’ll find a force fed 2-litre DOHC flat-four that puts out 195kW in factory form (which is de-tuned from Japanese spec). The stocko motor offers strong mid-range thrust and kicks the STi down the quarter mile in around 14 seconds flat.

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But Jim’s example can now embarrass a stock STi thanks to its much improved lung capacity. There’s a full-length APS (Air Power Systems) exhaust, APS cold air intake with K&N filter element and a free-flow APS front-mount air-to-air intercooler. This replaces the standard top-mount intercooler, which is negatively affected by under-bonnet heat. The factory STi blow-off valve is configured to recirculate air through the induction system.

The standard VF35 STi turbocharger is fine for applications up to around 250kW but it isn’t up to Jim’s ask. So the turbo was replaced by a bolt-on APS SR40 ball-bearing turbo. The new turbo is worked to 20 psi using a Blitz electronic boost controller.

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The fuel system is beefed-up with 630cc side-feed injectors used with the standard rails. A ‘600hp’ Bosch Motorsport fuel pump and custom surge tank ensure the injectors never run dry. The factory ignition system remains untouched.

Controlling fuel and ignition timing is a UniChip interceptor unit, which receives a load input from the factory airflow meter. The airflow meter is integrated into a new APS cast alloy housing.

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With the APS SR40 turbocharger set to deliver 20 psi boost, Jim’s STi can stomp out 212kW at all four wheels (measured on a Dyno Dynamics chassis dyno). This is around 80 percent more than the standard ATW output, so it’s reasonable to assume flywheel output is up to around 350kW. So far, the standard clutch and 6-speed gearbox have accepted this increased output without failure.

But we’re not so sure that record of reliability will continue...

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The latest mod to Jim’s STi - which was completed just before our photo shoot - can be found in the boot. A Nitrous Oxide Systems nitrous tank (complete with an electric warmer) stores N20 for a fogger nozzle which is mounted near the entry to the throttle. The nitrous is manually activated by a momentary switch on the gear knob. Damian from G-Tech can fit a choice of nozzles to vary the amount of extra grunt that comes from the nitrous and, at the time of writing, a 40hp (30kW) shot was given the nod.

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With around 350kW at the flywheel plus a 30kW shot of nitrous it seems you’re looking at 380kW - but Damian from G-Tech says is will be more than likely around 400kW (around 270kW at the wheels). This is due to the bonus cooling effect of nitrous.

Quarter mile times? Well, nobody knows for sure but you’d have to be looking at 12s.

In all likelihood, you wouldn’t recognise this as a 12 second machine when driving through traffic.

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Jim has fitted tasty Gestalt Virouge 20 inch rims with Nitto tyres and added gtech stickers down the side, across the bonnet and the back window. The stickers are coloured pink to highlight the original STi badges.

The body is lowered several inches but the standard STi dampers remain in operation, as do the Brembo brakes.

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Inside, you’ll find the factory STi wrap-around seats and all the sporty bits. Jim has also added a set of three STi Defi gauges in a beautifully integrated centre gauge pod. There are gauges for oil and water temperature, oil pressure and a separate boost gauge can be seen on the steering column. Oh, and there’s no need for a shift light when the factory already gives you one. Jim says a television screen will soon be making its way onto the scene.

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When Jim finally gets his licence back you can bet he will follow through on his plans to upgrade the engine internals and fit a bigger turbo – while also maintaining the car for Monday to Friday use. His weekend ride, if you must know, is a 2005 Yamaha R1 with aftermarket pipes and an upgrade ECU – a proven 10.4 second thrill machine!

Update:As this article goes online, Jim has had his licence reinstated and the STi has been further modified to deliver 320kW at the wheels. A 2.5 litre engine is next on the agenda - and the goal is now around 380kW at the treads!


G-Tech Performance                                                      +61 3 9305 2888


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